1. Field of the Invention
The present invention is generally directed to voltage-gated sodium channel Nav 1.3 splice variants. The invention further describes methods and compositions for the stable expression of such splice variants and methods of use of such compositions for identifying compounds that modulate the activity of sodium channels.
2. Background of the Related Art
The electrical activity of neuronal and muscle cells are governed by the activity of sodium channels on the plasma membrane of such cells. Rapid entry of sodium ions into the cell through such a channel causes depolarization of the membrane and generation of an action potential. Entry of sodium ions through sodium channels in response to a voltage change on the plasma membrane in excitable cells plays a functional role in the excitation of neurons in the central nervous system and the peripheral nervous system.
Sodium channels are voltage-gated transmembrane proteins that form ion channels within the membrane and have been the target of significant pharmocologic study, due to their potential role in a variety of pathological conditions. These sodium channels are responsible for the cellular uptake of sodium during the transmission of an electrical signal in cell membranes. The channels are members of a multigene family of proteins and are typically composed of a number of subunits. Typically, the pore of the channel is formed by the α-subunit and there are four accessory β-subunits, termed β1, β2, β3 and β3.
The β-subunits are involved in the modulation of the activity of sodium channel but the α-subunit is all that is required for the channel to form a functional ion pore. Co-expression of the β-subunits with the α-subunit has been shown to produce a more positive membrane potential. Further not all of the β-subunits are required, for example, it has been shown that the β3-subunits alone is sufficient to cause an increase in sodium current (Qu et al., Mol. Cell. Neurosci., 18(5):570-80, 2001).
The amino acid sequence of the sodium channel has been evolutionarily conserved. The channel is comprised of a signal polypeptide containing four internal repeats (domains I-IV). Each domain folds into six transmembrane α-helices or segments, of which five are hydrophobic and one is a highly-charged domain containing lysine and arginine residues (S4 segment). The highly-charged S4 segment is involved in the voltage gating properties of the sodium channel. The positively-charged side chains of the amino acids of the S4 segment are thought to be paired with the negatively-charged side chains of the other five segments such that upon membrane depolarization there is a shift in the position of one of the helices relative to the other resulting in an opening of the channel.
There are numerous variants of sodium channel α-subunit. These variants may be classified according to their sensitivity to tetrodotoxin (TTX). Those subunits that are inhibited by nanomolar quantities of TTX are classified as tetrodotoxin-sensitive channels, whereas those that require at least micromolar quantities of TTX for inhibition are referred to as tetrodotoxin-insensitive (1-5 micromolar). Those channels that require greater that 100 micromolar quantities of the TTX are termed tetrodotoxin-resistant. TTX is a toxin that blocks the conduction of nerve impulses along the axons and leads to paralysis. It binds to sodium channels and blocks the flow of sodium ions. It is believed that the positively-charged group of the toxin interacts with a negatively-charged carboxylate at the mouth of the channel on the extracellular side of the membrane thereby blocking the conductance of the pathway.
It has been noted that following nerve injury there is hyperexcitability (or an increased rate of spontaneous impulse firing in neurons) in peripheral sensory ganglia. It has been suggested that this hyperexcitability in neurons is due to altered sodium channel expression in some chronic pain syndromes (Tanaka et al., Neuroreport 1998; 9 (6): 967-72). Increased numbers of sodium channels leading to inappropriate, repetitive firing of the neurons have been reported in the tips of injured axons in various peripheral nervous tissues such as the DRG, which relay signals from the peripheral receptors to the central nervous system. Indeed, it has been noted that there is an increase in expression of an α1 Nav 1.3 subunit in axotomized DRG neurons together with elevated levels of α1 Nav1.1 and α1 Nav1.2 mRNAs (Waxman et al, Brain Res Mol Brain Res 1994; 22 (1-4): 275-89).
The peripheral input that drives pain perception is thought to depend upon the presence of functional voltage-gated sodium channels in peripheral nerves. It has been noted that there is a positive correlation between increased sodium channel expression in peripheral nerves. Some studies have also shown increased expression in association with neuropathic pain. In particular, it has been recognized that acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. Using antisense and knock-out studies, it has been shown that these sodium channels play a specialized role in pain pathways, and pharmacological studies (Wood et al., J Neurobiol., 61(1):55-71, 2004).
Most patients with traumatic spinal cord injury (SCI) report moderate to severe chronic pain that is refractory, or only partially responsive, to standard clinical interventions (Balazy, Clin J Pain 8: 102-110, 1992; Turner et al., Arch Phys Med Rehabil 82: 501-509, 2001). Experimental contusion SCI in rodents can produce long-lasting central neuropathic pain (Hulsebosch et al., J Neurotrauma 17: 1205-1217, 2000; Lindsey et al., Neurorehabil Neural Repair 14: 287-300, 2000; Hains et al., Neuroscience 116: 1097-1110, 2001; Mills et al., J Neurotrauma 18: 743-756, 2001). In spinally injured animals, alterations in electrophysiologic properties of dorsal horn neurons (Hao et al., Pain 45: 175-185, 1991; Yezierski and Park, Neurosci Lett 157: 115-119, 1993; Drew et al., Brain Res 893: 59-69, 2001; Hains et al., Neuroscience 116: 1097-1110, 2003a; Hains et al., Brain Res 970: 238-241, 2003b) are thought to contribute to changes in somatosensory responsiveness.
The TTX-sensitive Nav1.3-sodium channel is expressed at relatively high levels in embryonic dorsal root ganglion (DRG) neurons but is barely detectable in adult DRG neurons and its expression is decreased in the adult spinal cord and CNS throughout development. However, the expression of Nav1.3 mRNA and protein is markedly upregulated in DRG neurons of adult rats after axotomy of peripheral projections, after chronic constriction injury, and after tight spinal nerve ligation. This produces a rapidly repriming TTX-S current that permits neuronal firing at higher than normal frequencies. It has been shown that an increase in expression of Nav1.3, similar to the changes in DRG neurons after peripheral axotomy, takes place in lumbar dorsal horn sensory neurons after SCI. It has further been shown that knock-down or reduction of expression of Nav1.3 mRNA and protein results in a reduction in hyperexcitability of dorsal horn sensory neurons and pain-related behaviors in animals (Hains et al., J. Neurosci., 23(26):8881-8892, 2003).
There are various sodium channels that remain to be characterized. Identification of such channels will facilitate further studies and identification and characterization of further isotype-specific antagonists of sodium channel blockers. Such sodium channel blockers or antagonists will be useful in the management of pain. Preferably, such analgesic agents are such that treatment of pain is facilitated without having deleterious side effects due to cardiac, central nervous system or neuromuscular complications.